TRANSPORT IN PLANTS
XYLEM AND PHLOEM
Functions of Xylem & Phloem

Plants contain two types of transport vessel:
o
Xylem vessels – transport water and minerals (pronounced: zi-lem) from the roots
to the stem and leaves
o
Phloem vessels – transport food materials (mainly sucrose and amino acids) made
by the plant from photosynthesising leaves to non-photosynthesising regions in the
roots and stem (pronounced: flow-em)

These vessels are arranged throughout the root, stem and leaves in groups called vascular
bundles
Vascular tissue in a dicotyledonous plant
Exam Tip
If you are asked to identify the xylem or phloem in a diagram showing a cross-section of a root,
stem or leaf just remember that xylem is always on the inside and phloem is always on the outside.
ROOT HAIR CELLS

Root hairs are single-celled extensions of epidermis cells in the root

They grow between soil particles and absorb water and minerals from the soil

Water enters the root hair cells by osmosis

This happens because soil water has a higher water potential than the cytoplasm of the root
hair cell
S
tructure of the root
How the Large Surface Area of a Root Hair Cell is Useful

The root hair increases the surface area of the cells significantly

This large surface area is important as it increases the rate of the absorption of water by
osmosis and mineral ions by active transport
Pathway of Water through Root to Leaf

Osmosis causes water to pass into the root hair cells, through the root cortex and into the
xylem vessels:
Pathway of water into and across a root

Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll
cells

So the pathway is:
root hair cell → root cortex cells → xylem → leaf mesophyll cells

The pathway can be investigated by placing a plant (like celery) into a beaker of water that
has had a stain added to it (food colouring will work well)

After a few hours, you can see the leaves of the celery turning the same colour as the dyed
water, proving that water is being taken up by the celery

If a cross-section of the celery is cut, only certain areas of the stalk is stained the colour of
the water, showing that the water is being carried in specific vessels through the stem these are the xylem vessels
Investigating water movement in plants using a stain
TRANSPIRATION
What is Transpiration?

Water travels up xylem from the roots into the leaves of the plant to replace the water that
has been lost due to transpiration

Transpiration is defined as the loss of water vapour from plant leaves by evaporation of
water at the surfaces of the mesophyll cells followed by diffusion of water vapour through
the stomata

Xylem is adapted in many ways:
o
A substance called lignin is deposited in the cell walls which causes the xylem cells
to die
o
These cells then become hollow (as they lose all their organelles and cytoplasm)
and join end-to-end to form a continuous tube for water and mineral ions to travel
through from the roots
o

Lignin strengthens the plant to help it withstand the pressure of the water movement
Movement in xylem only takes place in one direction - from roots to leaves (unlike phloem
where movement takes place in different directions)
Water uptake, transport and transpiration
Transpiration in plants

Transpiration has several functions in plants:
o
transporting mineral ions
o
providing water to keep cells turgid in order to support the structure of the plant
o
providing water to leaf cells for photosynthesis
o
keeping the leaves cool (the conversion of water (liquid) into water vapour (gas) as
it leaves the cells and enters the airspace requires heat energy. The using up of heat
to convert water into water vapour helps to cool the plant down)
How does Transpiration Occur?

Evaporation takes place from the surfaces of spongy mesophyll cells

The many interconnecting air spaces between these cells and the stomata creates a large
surface area

This means evaporation can happen rapidly when stomata are open
THE TRANSPIRATION STREAM
How is the Transpiration Stream Created?

Water molecules are attracted to each other by cohesion - creating a continuous column of
water up the plant

Water moves through the xylem vessels in a continuous transpiration stream from roots to
leaves via the stem

Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves

As water molecules are held together by cohesive forces (each individual molecule ‘pulls’
on the one below it), so water is pulled up through the plant

If the rate of transpiration from the leaves increases, water molecules are pulled up the
xylem vessels quicker
Wilting

If more water evaporates from the leaves of a plant than is available in the soil to move
into the root by osmosis, then wilting will occur

This is when all the cells of the plant are not full of water, so the strength of the cell walls
cannot support the plant and it starts to collapse
Investigating Factors that Affect Transpiration
Investigating the role of environmental factors in determining the rate of transpiration from a leafy
shoot

Cut a shoot underwater to prevent air entering the xylem and place in tube

Set up the apparatus as shown in the diagram and make sure it is airtight, using vaseline to
seal any gaps

Dry the leaves of the shoot (wet leaves will affect the results)

Remove the capillary tube from the beaker of water to allow a single air bubble to form
and place the tube back into the water

Set up the environmental factor you are investigating

Allow the plant to adapt to the new environment for 5 minutes

Record the starting location of the air bubble

Leave for a set period of time

Record the end location of air bubble

Change the light intensity or wind speed or level of humidity or temperature (only one whichever factor is being investigated)

Reset the bubble by opening the tap below the reservoir

Repeat the experiment

The further the bubble travels in the same time period, the faster transpiration is
occurring and vice versa
Investigating transpiration rates using a photometer

Environmental factors can be investigated in the following ways:
o
Temperature : Temperature of room (cold room and warm room)
o
Humidity : Spray water in plastic bag and wrap around plant
Exam Tip
Remember when designing an investigation to ensure a fair test you must keep all factors the same
other than the one you are investigating.
Temperature & Humidity on Transpiration Rate
Factors affecting rate of transpiration:
Transport of Food

The soluble products of photosynthesis are sugars (mainly sucrose) and amino acids

These are transported around the plant in the phloem tubes which are made of living cells
(as opposed to xylem vessels which are made of dead cells)

The cells are joined end to end and contain holes in the end cell walls (called sieve plates)
which allow easy flow of substances from one cell to the next

The transport of sucrose and amino acids in phloem, from regions of production to regions
of storage or use, is called translocation

Transport in the phloem goes in many different directions depending on the stage of
development of the plant or the time of year; however dissolved food is always transported
from source (where it’s made) to sink (where it’s stored or used):
o
During winter, when many plants have no leaves, the phloem tubes may transport
dissolved sucrose and amino acids from the storage organs to other parts of the
plant so that respiration can continue
o
During a growth period (eg during the spring), the storage organs (eg roots) would
be the source and the many growing areas of the plant would be the sinks
o
After the plant has grown (usually during the summer), the leaves are
photosynthesizing and producing large quantities of sugars; so they become the
source and the roots become the sinks – storing sucrose as starch until it is needed
again
Translocation through the phloem
Comparison between xylem and phloem tissue: